Generally, a liquid developer for electrostatic photography comprises a carrier liquid having a high electric resistance (10.sup.9 to 10.sup.15 .OMEGA..cm), positively or negatively charged toner particles, a fixing resin for giving fixability to the toner particles, a dispersant for dispersing stably the toner particles, a charge-controlling agent for charging the toner particles positively or negatively, and various additives. A charge-controlling agent gives positive or negative distinct charge polarity to toner particles; it also controls the charged amount, and is an essential component for liquid developers.
Since the charge of toner particles greatly influences the image obtained after the development processing, strong efforts are made to control the charge stably. To adjust the charge, roughly classified, two methods are known. The first method comprises covering the surface of toner particles with a material that can be ionized or that can adsorb ions. As the material used for this purpose, oils and fats, such as linseed oil and soybean oil, alkyd resins, halogenated polymers, aromatic carboxylic acids disclosed in JP-B ("JP-B" means examined Japanese patent publication) No. 5944/1976, acid group-containing water-soluble dyes disclosed in JP-B No. 12869/1981, and oxidized condensates of aromatic polyamines disclosed in JP-A ("JP-A" means unexamined published Japanese patent application) No. 120629/1975 are known. Further, for example, pigment/polymer compositions (e.g., grafted carbon) which are obtained by polymerizing a polar monomer in the presence of a pigment and are disclosed in JP-B Nos. 6157/1971, 6151/1971, and 13584/1976, and a polymer electrolyte disclosed in JP-B No. 6354/1978, are known. In this method, since the toner particles themselves have polar groups, the amount of the ion component contained in the carrier liquid can be reduced to produce a developer good in image quality, but there are such problems as that it is difficult to adjust the charged amount finely and that a change of the charged amount with the lapse of the time is conspicuous in some cases of substances used.
The second method allows a substance to be present which dissolves in a carrier liquid and can transfer ions to or from toner particles. As examples of such a substance, a metal naphthenate, such as nickel naphthenate and cobalt naphthenate; a metal soap, such as cobalt 2-ethylhexanoate; a metal sulfonate, such as calcium dodecylbenzenesulfonate, a petroleum-type metal sulfonate, and a metal salt of a sulfosuccinate; a lecithin; a polyvinylpyrrolidone resin; a polyamide resin; a sulfonic acid-containing resin described in JP-B No. 24944/1981; and a hydroxybenzoic acid derivative described in JP-A No. 139753/1982, are known. Quaternary ammonium soluble copolymers described in JP-A Nos. 31739/1979, 137960/1984, and 39059/1986; a metal salt such as a nickel salt of an amino acid derivative, described in JP-A No. 50951/1986; a semialkylamide compound of a diisobutylene/maleic acid copolymer described in JP-B No. 26596/1974: and itaconic anhydride derivatives and hemi-maleic acid amides described in JP-A Nos. 173558/1985, 179750/1985, and 182447/1985, are also known.
Although the second method is generally used, since the charged amount is easily controlled by a charge-controlling substance, the electric resistance of the liquid developer is apt to be lowered because the added charge-controlling substance is generally liable to be ionized. As a result, the optimum amount of charge-controlling substance to be added is restricted extremely, and when the added amount exceeds the optimum amount, such adverse effects as a decrease of the image density and flow-out of the image occur. On the other hand, if the amount of the charge-controlling substance to be added is small, there arises the problem that collapse of shadow details of dots and fringes surrounding the image (leading to the occurrence of doublets) occur. In this case, it becomes an important technique for the charged amount to be increased without increasing the amount to be added. Further, when many sheets are developed, the concentration of the charge-controlling agent changes or the definition of the image changes. Furthermore, after the liquid developer is prepared, in some cases, the magnitude of the charge changes with time, to adversely affect the image quality or to increase the settling of the toner particles. Further, there are even some substances that denature due to oxidation or the like during storage and lose charge-controllability.
Generally, toner particles contain a colorant, such as a pigment and a dye, in order to make the image visible. It is known that the charge of the toner particles is noticeably influenced by some kinds of these colorants, and also that the charging ability of the colorant is used to charge toner particles. On the other hand, in an application wherein only ink adhesion is required, such as for printing plates, or when the toner image section is used as a resist and the non-image area is dissolved out to produce a printing plate, a colorant is not necessarily required, and it is rather desirable not to contain a colorant because a colorant soils a printed product or lowers the resist property. Toner particles not containing a colorant, that is, toner particles mainly consisting of a polymer, are difficult to be charged and, for example, polar groups are generally introduced to the polymer particles. In this case, the polymer particles are attended with the same problems as those of the first method.